A Major New Fossil Deposit, with a Note on Taxonomic Caution

PNAS yesterday carried breaking news by Rust and colleagues of extensive new fossil Indian amber deposits dating to about 50 million years ago and holding exquisitely preserved fossils. How important is this find?

It’s huge.

Not only does the discovery add an older record to bridge the excellent Dominican (15 mya) and Baltic (40 mya) deposits with the various and spotty Cretaceous deposits (65-140 mya ), but it provides insight on the history of the Indian subcontinent as it migrated from the south. The fossils will give biologists grist for years of testing hypothesis about what happened to India on its northward journey, and for examining broader patterns of evolution in the contained taxa. This is big, exciting stuff.

But enough praise for the research. I always find some nitpick to rant about, and Rust et al are no exception.

On the surface, the PNAS paper indicates the Indian paleofauna had non-intuitive connections to northern Europe and Australasia. Or, in the authors’ words, “India just prior to or immediately following contact shows little biological insularity.”

But this conclusion depends very much on the integrity of the taxonomic identifications. And I think they’ve botched their exemplar ant:

Fig 2B: a Myrmicine?

From the paper:

Despite the biased comparisons to the Baltic amber paleobiota, phylogenetic affinities of the identified insects in Cambay amber also lie with Recent taxa from southeastern Asia and Australasia… In ants, a highly sculptured, distinctive myrmecine [sic] (Fig. 2B) is closely related to a small group of Recent southeast Asian and Australian genera, Lordomyrma, Meyriella [sic], and Austromorium.

The fact that myrmicine and Mayriella are misspelled is a fat red flag that the specimen wasn’t examined by any ant specialists. Indeed, this ant shares only a suite of rather plastic traits with the fingered Australasian myrmicines: coarse areolate sculpture, propodeal spines, and the like. These are superficial characters that aren’t ideal for deeper phylogenetic inference.

Let me offer an alternate interpretation.

The mystery ant is not a myrmicine at all, Australian or otherwise. It’s an agroecomyrmecine. This is an ancient, presumably carnivorous lineage known only from Baltic amber (Agroecomyrmex), the Florissant fossil beds of Colorado (Eulithomyrmex), and a handful of rare extant Neotropical species (Tatuidris).

I just did a set of quick-and-dirty measurements from the mystery specimen and from several other Antweb specimens of head length, eye placement on head, and mesosomal length:

The Mystery Ant is more similar in eye placement and mesosomal proportions to Agroecomyrminae than to Rust et al’s hypothesized Australasian relatives

Several traits indicate an agroecomyrmecine affinity:

The position of the compound eye high on the head

The relatively broad and stubby mesosoma

The bulky post-petiole (=abd III) with a broad attachment to Abd IV

An apparently expanded Abd 4 tergite (not easy to see in the fossil).

Wheeler’s illustration of the Agroecomyrmex types from Baltic amber

If true, this alternate identity of the fossil is even more exciting. Agroecomyrmecinae is one of the oddest, rarest, and least understood of all ant lineages. An excellently-preserved fossil on a 50 million year old Indian subcontinent adds considerably to our knowledge of the group. Of course, this identification also nixes Rust et al’s Australasian-India connection, but I’ll happily trade that for another agroecomyrmecine.

My taxonomic whinging aside, though, the Cambay amber is a major discovery. Keep an eye out for the more detailed descriptions of the contained fossils that are sure to come.

At first I also thought the ant is nothing like any of the genera they compare it to. My first impression was a tiny Meranoplus-like thing, although still quite different. Your Agroecomyrmecinae argument got me convinced! Too bad that abdominal segment III-IV junction is a bit obscured, but now that you mentioned it, they do seem to be rather broadly attached.

The antennae on the mystery specimen seems to have no distinct club, and this trait seems to be shared with Lordomyrma and Austromorium, whereas Tatuidris (at least the specimen on Antweb) has a distinct (2-segmented?) club. How does this trait compare in importance to the others? Also, from the way I see it, the post-petiole’s attachment to the gaster is almost exactly as broad as that seen in Austromorium hetericki.

That’s a good point. I spent some time looking at the antennae to see if there was anything to be gleaned. But the antennae within Agroecomyrmecinae- and Myrmicinae- are variable enough that I didn’t think it would be diagnostic. Consider Agroecomyrmex itself, with a much more generalized antenna than the modern and highly derived Tatuidris.

You’re right though that we’re really playing a fool’s game by picking and choosing single characters. I used ones I consider diagnostic for Agroecomyrmecinae, and that are visible in a single lateral view of the fossil. What is needed is a proper phylogenetic analysis based on a thorough examination of the specimen rather than a photograph.

While I’m only, say, 60% on this thing being an Agroecomyrmecinae, I can see enough to know that the ant is surely not related to Lordomyrma/Mayriella, etc. At least, not more to those than to any other myrmicine.

Oh, Ok. I didn’t know how variable something like an antennae is. I look forward to a more certain classification, or at least description of its relationship to other ants, if such a thing is possible!